Gas sensors have been in use for some time to sense various gases such as hydrogen, oxygen, carbon monoxide, etc. One form of a gas sensor is an electrochemical cell that uses a catalytic electrode so that the gas to be detected is either oxidized or reduced with the exchange of electrons. The flow of current due to the oxidation or reduction of the gas is then detected as a measure of the concentration of the gas to be detected.
However, a known problem with gas sensors is that they lose sensitivity over time. For example, the working life of an electrochemical cell is determined by the activity of the cell's catalytic electrode that is used to detect gas within the sensor. This activity is gradually reduced over time by contaminant gases and poisons such that the sensitivity of the sensor drifts downward.
Other types of gas sensors, such as pellistor sensors, biomimetic sensors, and tin oxide sensors that may be formed as thin film, thick film, sintered or MOSFET devices, may have similar problems.
If the instrument into which the gas sensor is built is calibrated regularly, this downward sensitivity drift can be compensated for by adjusting the gain of the gas sensor, and any faulty gas sensors can be replaced immediately. However, if the instrument is in a difficult position to service, or if calibration of the gas sensor is not otherwise freely available, it is often impossible to confirm that the gas sensor is functioning correctly. Therefore, as the gas sensor reaches the end of its working life, the output of the sensing cell may be insufficient to generate an alarm condition. As a result, a situation could arise where toxic levels of gas are present, but the gas sensor is incapable of providing the requisite warning.
A substantial effort has been invested in determining a method by which the function of a gas sensor, such as an electrochemical cell, can be checked without the need for an externally generated calibration gas. For example, it has been proposed to use additional electronic components in order to check conductive pathways through the gas sensor. While such methods can uncover broken connections, they do not provide any information on the condition of the electrodes in terms of their ability to react with the gas to be detected.
When external gas sources are used, gas detectors for industrial applications are normally calibrated to correct for drift. Toxic gas detectors are normally calibrated to measure around the Occupational Exposure Level, which for most toxic gases will be less than 50 ppm, an extremely low level. Because of the difficulty in preparing gas/air mixtures at this dilution, because some toxic gases such as hydrogen sulfide and sulfur dioxide are readily absorbed by the materials used to make the calibration gas cylinder housings, and because the stability of these mixtures can be a problem, calibration gas cylinder have a limited shelf life.